A Computer Simulation of Competition Among Five Sympatric Congeneric Species of Xanthid Crabs

In Hawaii, five species of xanthid crabs of the genus Trapezia are commensals of the branching coral Pocillopora meandrina var. nobilis. As adults, crabs are found in heterosexual pairs. Intraspecific agonistic interactions typically restrict host occupancy to one pair per species but multiple species combinations are common. Though niche overlap among the species is nearly complete, they exhibit statistically different host size requirements. Field observations suggested that the distribution patterns of the species are influenced by interference competition during host selection. A stochastic computer model of host selection produces species distributions similar to those observed in the field. In the absence of evidence consistent with alternative hypotheses, this suggests that interference competition resulting from random encounters during host selection by adults is sufficient to account for site variability in the distribution patterns of species of Trapezia. Simulation experiments were conducted to assess the relative importance of the potential sources of competitive pressure to each species of Trapezia. The suitability of the hosts available at each study site to each species was estimated 1) assuming the presence of competitors and 2) assuming the hosts were uncolonized. The decrease in suitability of the hosts due to the presence of competitors was used as an index of competitive pressure on the species. There was no significant correlation between the degree of niche overlap and the intensity of competition between species. Caution is advised in using niche overlap measures to estimate competition intensity.